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Assign a Not-So-Stubby Area (NSSA)
The OSPF Not So Stubby Area (NSSA) feature enables you to configure OSPF areas that provide the benefits of stub areas, but that also
are capable of importing external route information. OSPF does not flood external routes from other areas into an NSSA, but does
translate and flood route information from the NSSA into other areas such as the backbone.
NSSAs are especially useful when you want to summarize Type-5 External LSAs (external routes) before forwarding them into an OSPF
area. The OSPF specification (RFC 2740) prohibits summarization of Type-5 LSAs and requires OSPF to flood Type-5 LSAs
throughout a routing domain. When you configure an NSSA, you can specify an address range for aggregating the external routes that
the NSSAs ABR exports into other areas.
Since the NSSA is partially "stubby" the ABR does not flood external LSAs from the backbone into the NSSA. To provide access to the
rest of the Autonomous System (AS), the ABR generates a default Type-7 LSA into the NSSA.
Configuring an NSSA
Using the area nssa command, you can block the generation of type-3 and type-7 LSAs into an NSSA. This command also provides an
option to configure the NSSA translator role.
Configuration examples
The following example creates an NSSA area with an area-id 100. If the router is an ABR then a type-3 summary LSA will be originated
into the NSSA area and if the router is an ASBR then type-7 NSSA external LSA will be generated into NSSA area with a default external
metric value of 10. The routers NSSA translator role will be set to candidate and it will participate in NSSA translation election.
device(config-ospf6-router)# area 100 nssa
The following example modifies the NSSA area 100 wherein type-7 NSSA external LSA will not be originated into NSSA area. But the
type-3 summary LSAs will still be originated into NSSA area.
device(config-ospf6-router)# area 100 nssa no-redistribution
The following example modifies the NSSA area 100 wherein origination of type-3 summary LSAs (apart from type-3 default summary)
will be blocked into NSSA area. The CLI works in incremental fashion and the origination of type-7 LSA will be continued to be blocked
as 'no-redistribution' option was enabled in the previous command.
device(config-ospf6-router)# area 100 nssa no-summary
The following example modifies the NSSA area 100 wherein origination of the self-router acts as NSSA translator. The generation of
type-3 & type-7 LSA will still be blocked into NSSA area.
device(config-ospf6-router)# area 100 nssa translator-always
The following example modifies the NSSA area 100 wherein origination of type-3 summary will be allowed, but origination of type-7
LSA will still be blocked. Also the self-router will still act as NSSA translator-always.
device(config-ospf6-router)# no area 100 nssa no-summary
Although the NSSA configuration can be done in an incremental fashion during show-run, all the configuration options will be displayed
in just one line. For example, the output of the
show run would be:
device(config-ospf6-router)# area 100 nssa no-redistribution translator-always
The following example deletes the NSSA area 100.
device(config-ospf6-router)# no area 100
Syntax: [no] area area-id nssa [[ stub-metric ] [ default-information-originate [ metric metric-value | metric-type type-value ]] [ no-
summary ] [ no-redistribution ] [ translator-always ] [ translator-interval stability-interval ]]
Configuring OSPFv3
FastIron Ethernet Switch Layer 3 Routing
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